1. Field of the Invention
This invention relates to molybdenum-catalyzed coal liquefaction processes, and in particular to the recovery of molybdenum from coal liquefaction residues to permit reuse of the molybdenum to catalyze additional coal liquefaction.
Current efforts to develop alternate energy sources to petroleum for fuels and hydrocarbon feedstocks have included investigation of processes for converting coal into a variety of products. One type of process is known as coal liquefaction. In coal liquefaction, coal is typically pulverized and treated with one or more catalysts, typically by spraying an aqueous solution, emulsion, or suspension of the catalytic agent onto the coal. The thus prepared coal is then reacted with hydrogen at temperatures of about 1000.degree. F. and pressures of about 1000 psi to produce a mixture of hydrocarbon liquids and gases and a tarry carbonaceous residue containing the metallic component of the catalyst. The tarry residue is then gasified, that is, treated at elevated temperature and pressure to generate H.sub.2 and CO and to recover additional BTU value from the tar as a gas stream, leaving an ash residue. The H.sub.2 is then used in the coal hydrogenation reactor for liquefaction.
It is desirable to employ a molybdenum-based catalyst in coal liquefaction because, generally speaking, catalyzing the coal liquefaction reaction with molybdenum improves the quantity and quality of liquid fuel products recovered. That is, more of the liquid products of molybdenum-catalyzed coal hydrogenation can be used as transportation fuels, e.g., gasoline, jet fuel, or Diesel fuel, whereas most of the liquid products of coal hydrogenation carried out without molybdenum catalysts are generally limited to use as fuel for power plants.
The economic value of the molybdenum in such catalysts makes recovery of the molybdenum from the coal liquefaction residue highly desirable, so that the molybdenum is not discarded with the residue. The molybdenum should advantageously be recovered in a form which makes the valuable properties of the molybdenum readily available; more advantageously, the molybdenum should be recovered in a form which can be applied to coal to catalyze coal liquefaction in a continuous process.
Prior to the present invention, however, recovery of the molybdenum from a coal liquefaction residue has not been feasible. The ash residue produced in the gasifier stage mentioned above is initially molten, and solidifies on cooling into a glassy solid. The molybdenum is finely disseminated in metallic or reduced oxide form throughout the glassy matrix. Previously known processes for recovering molybdenum have not been directed to this type of molybdenum-bearing source material. Thus, there is a need for a process for recovering molybdenum from such residues, and especially for recovering the molybdenum in a form in which the molybdenum can be readily reused in the coal liquefaction process.
2. Description of the Prior Art
The desirability of recovering the molybdenum from spent catalysts is illustrated by U.S. Pat. No. 4,087,510, which discloses a process for recovering molybdenum and vanadium from a spent petroleum hydro-desulfurization catalyst. Hydro-desulfurization converts sulfur in high-sulfur petroleum fractions to hydrogen sulfide. The conversion is catalyzed by a catalyst typically comprising compounds of molybdenum and cobalt deposited on an aluminum oxide support. The spent catalyst comprises molybdenum disulfide deposited on the surfaces of the aluminum oxide support, which is also contaminated with vanadium and cobalt sulfides, hydrocarbon oil, and carbon. This material is quite different from the molybdenum-bearing coal liquefaction residue that is treated in the present invention, since the coal residue contains metallic or reduced oxide molybdenum entrapped within solid, glassy ash. Thus, the prior patented process is not directly pertinent to the process of the present invention.
According to U.S. Pat. No. 4,087,510, the spent hydro-desulfurization catalyst is mixed with solid alkali metal carbonate, and the mixture is heated in the presence of air to a temperature between about 650.degree. C. and about 850.degree. C. for one to two hours. The heated product is then leached with water to dissolve vanadium and molybdenum compounds. The patentee states that molybdate can be precipitated as ammonium tetramolybdate "by using hydrochloric acid at a pH of about 2.5", but there is no disclosure of processing steps to prepare the molybdenum for reuse in any catalytically effective form.
U.S. Pat. No. 4,075,277 and U.S. Pat. No. 4,075,278 relate to the recovery of molybdenum from spent catalysts comprising molybdenum and other metals on an alumina support. The spent catalyst is impregnated with an aqueous solution of sodium carbonate containing a stoichiometric excess of sodium carbonate of up to about 10%, and then roasted at 600.degree. C. to 800.degree. C. The roasted material is cooled and then leached in water to dissolve sodium molybdate. The leach liquor is treated with successive additions of nitric acid to precipitate molybdic acid. The latter patent concerns a modification of the process in which the spent catalyst is treated with CO.sub.2 gas to prevent deposits from building up in the system. These patents describe the treatment of material which is significantly different from the coal liquefaction residue and do not produce an ammonium-polymolybdate product.
Several patents assigned to the assignee of the present application relate to the recovery of molybdenum as an ammonia-molybdenum compound, but do not suggest the process of the present invention. One, U.S. Pat. No. 3,763,303, discloses a process for extracting molybdenum values from spent catalysts, such as spent epoxidation catalysts, involving a series of extraction steps with e.g. potassium hydroxide or ammonium hydroxide, followed by acidification to precipitate "ammonium molybdenum compounds". Others, namely U.S. Pat. No. 3,829,550, U.S. Pat. No. 3,848,049, and U.S. Pat. No. 4,079,116, disclose leaching solid MoO.sub.3 with aqueous ammonium hydroxide to form an aqueous solution containing dissolved ammonium-molybdate compounds. These four patents do not disclose the steps in the presently claimed invention for recovering an ammonium-polymolybdate precipitate from an alkali molybdate leach liquor, and they do not suggest the claimed process for treating coal liquefaction residues to recover molybdenum values in catalytically effective form.